Cool thermal energy storage is increasingly being used in building air conditioning systems. The major advantage of cool thermal energy storage is that it reduces the severity of intermittent peak air conditioning loads. That is, off-peak electrical utility periods may be used to operate the cool storage equipment and thereby alleviate the severity of the peak air conditioning loads. The shift of electrical energy use to utility off-peak periods reduces the customer's demand charges, leading to a reduction in electric bills. In addition, the load shift reduces the utility's system peak demand, thereby improving operating efficiency and reducing costs.
Several methods are currently used for sensible and latent cool thermal energy storage. Dynamic ice harvesters are one technique for latent heat storage. In typical ice harvesting systems, ice is formed on the exterior surface of a heat transfer surface and periodically removed by means of a defrost harvesting cycle, which melts the ice adjacent to the heat transfer surface, thereby allowing all of the ice to be removed from the heat transfer surface. Overall efficiency of the system is adversely effected by the defrost harvesting cycle, which may utilize up to about 20% of the energy input to the system. Mechanical harvesting techniques, such as scrapping ice from a surface, may also be used. However, such techniques also require additional energy.
U.S. Pat. No. 4,907,415 (the '415 patent), owned by the assignee of the present invention and expressly incorporated by reference herein, describes an improved system in which a "self release" harvesting technique is used to efficiently obtain ice for use in a cool thermal energy storage system. The apparatus of the '415 patent includes a thermal storage solution and a heat exchanger.
The thermal storage solution comprises water and a mixture of electrolytes and/or non-electrolytes, such as a 30/70 calcium acetate-magnesium acetate mixture or ethylene glyclol, respectively. When processed by the heat exchanger, the thermal storage solution results in a mixture of liquid and ice crystals, forming a "slush". "Slush" is defined as a soft mass consisting of a mixture of ice crystal solids and liquid. The slush is soft compared to a solid formed from freezing substantially pure water. As will be described below, the solution results in low adhesion forces (compared to substantially crystalline ice) between the ice crystals and the heat exchanger surface. The term "adhere" is used as a generic term for all forces tending to cause a mass consisting of a mixture of solids and a liquid to be attracted to a surface. Low cohesion forces cause the slush to be soft. As used herein, the term "cohere" is used as a generic term for all forces tending to cause ice crystals and a liquid to be attracted to each other.
FIG. 1 illustrates one embodiment of the invention disclosed in the '415 patent. The tank 20 holds the thermal storage solution 22. The liquid portion of the thermal storage solution is removed from the tank 20, using any convenient technique, and is directed through a conduit 24 by a pump 26. The output of the pump 26 is in turn directed by conduit 28 to distributors 30 and 32.
Liquid 33, generally indicated by arrows, from the distributors 30 and 32 is directed to flow down the substantially vertical (downwardly extending) heat exchanger surfaces 34 and 36. Cold refrigerant flows through each of the heat exchanger surfaces, 34 and 36, entering by way of input conduit 38 and exiting by output conduit 40.
The temperature of the refrigerant is selected such that as the liquid 33 flows down the heat exchanger surfaces, 34 and 36, portions of the liquid 33 solidify to form ice crystals, illustrated as reference numeral 42. The accumulated ice crystals 42 interact with the liquid portion 33 of the thermal storage solution, the heat exchanger surfaces 34 or 36, and forces of gravity. The magnitude of these interactions increases as the accumulated ice crystals 42 increase in size and/or thickness. As these interactions become sufficient to overcome adhesion and/or cohesion forces, portions or all of the accumulated ice crystals 42 release from the heat exchanger surfaces 34 and 36 and are accumulated in tank 20. As previously indicated, the tank 20 holds the thermal storage solution, which may be predominantly slush or liquid, depending on the operating history of the system.
It would be desirable to improve upon the technology disclosed in the '415 patent. Specifically, it would be desirable to improve the thermal storage solution such that accumulated ice crystals could be more readily removed from a heat exchanger surface.